Dynamic Embrittlement — Time–dependent Quasi–brittle Intergranular Fracture at High Temperatures

Krupp, Ulrich

doi:10.1179/174328005x14320

Cited by 85 publications

(89 citation statements)

References 89 publications

(116 reference statements)

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“…This appears to contradict much of the literature available (see for example, Refs. 27,36,37 ), which infers the greater penetration depths from stress-assisted grain boundary diffusion. The premise of this argument states that the hydrostatic stress increases the oxygen diffusivity.…”

Section: Discussionmentioning

confidence: 99%

Oxidation of a Commercial Nickel-Based Superalloy under Static Loading

Foss

Hardy

Child

et al. 2014

JOM

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The current demands of the aviation industry for increased gas-turbine efficiency necessitate higher turbine entry temperatures, requiring that alloys exhibit superior oxidation resistance. The synergistic effects of oxidation and mechanical stresses pose a complex issue. The purpose of the current research was to examine the effects of stress on the oxidation and oxygen transport in a commercial nickel-based superalloy. Fine grain RR1000 in both polished and shot-peened conditions was studied for classic (zero load) and statically loaded conditions using integrated two-stage isotopic tracing combined with focusedion-beam secondary ion mass spectrometry (FIB-SIMS). Cr 2 O 3 external oxide formed with semicontinuous TiO 2 above and below. Preferential grain boundary Al 2 O 3 internal oxide formation, c¢-dissolution, and recrystallization occurred subsurface. Oxidation mechanisms were dominated by anionic/cationic growth in the external oxide with inward oxygen transport, initially through the partially unprotective external oxide, then along internal oxide/ alloy interfaces. Loading did not influence the oxidation products formed but did bring about expedited oxidation kinetics and changes to the oxide morphology. The oxygen diffusivity D O * (910 À13 cm 2 s À1 ) ranged from 0.39 for the polished alloy to 3.7 for the shot-peened condition under compressive stress. Arguably, the most significant effects took place in the subsurface regions. Increased oxidation kinetics were attributed to the development of fast cation diffusion paths as the alloy deformed by creep.

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Section: Discussionmentioning

confidence: 99%

Oxidation of a Commercial Nickel-Based Superalloy under Static Loading

Foss

Hardy

Child

et al. 2014

JOM

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“…Intergranular cracking is not due to the formation of massive oxidation products along the grain boundaries. The mechanism is better described as nano scaled dynamic embrittlement, where oxygen diffuses into highly stressed grain boundaries at the crack tip and causes decohesion [18,19].…”

Section: Discussionmentioning

confidence: 99%

Influence of overloads on dwell time fatigue crack growth in Inconel 718

Saarimäki

Moverare

Eriksson

et al. 2014

Materials Science and Engineering: A

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Inconel 718 is one of the most commonly used superalloys for high temperature applications in gasturbines and aeroengines and is for example used for components such as turbine discs. Turbine discs can be subjected to temperatures up to ∼ 700 • C towards the outer radius of the disc. During service, the discs might start to develop cracks due to fatigue and long dwell times. Additionally, temperature variations during use can lead to large thermal transients during start-up and shutdown which can lead to overload peaks in the normal dwell time cycle. In this study, tests at 550 • C with an overload prior to the start of each dwell time, have been performed. The aim of the investigation was to get a better understanding of the effects of overloads on the microstructure and crack mechanisms. The microstructure was studied using electron channelling contrast imaging (ECCI). The image analysis toolbox in Matlab was used on cross sections of the cracks to quantify: crack length, branch length, and the number of braches in each crack. * jonas.saarimaki@liu.se +46 13 28 11 93Preprint submitted to Material Science & Engineering A June 2, 2014It was found that the amount of crack branching increases with an increasing overload and that the branch length decreases with an increasing overload.When the higher overloads were applied, the dwell time effect was almost cancelled out. There is a strong tendency for an increased roughness of the crack path with an increasing crack growth rate.

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“…(22) with the potential defined by Eqs. (23)(24) and taking into account the definition of effective opening displacement, Eq. (19).…”

Section: Damaged Interface: Cohesive Traction-separation Lawsmentioning

confidence: 99%

A three-dimensional cohesive-frictional grain-boundary micromechanical model for intergranular degradation and failure in polycrystalline materials

Benedetti

Aliabadi

2013

Computer Methods in Applied Mechanics and Engineering

116

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In this study, a novel three-dimensional micro-mechanical crystal-level model for the analysis of intergranular degradation and failure in polycrystalline materials is presented. The polycrystalline microstructures are generated as Voronoi tessellations, that are able to retain the main statistical features of polycrystalline aggregates. The formulation is based on a grain-boundary integral representation of the elastic problem for the aggregate crystals, that are modeled as threedimensional anisotropic elastic domains with random orientation in the three-dimensional space. The boundary integral representation involves only intergranular variables, namely interface displacement discontinuities and interface tractions, that play an important role in the micromechanics of polycrystals. The integrity of the aggregate is restored by enforcing suitable interface conditions, at the interface between adjacent grains. The onset and evolution of damage at the grain boundaries is modeled using an extrinsic non-potential irreversible cohesive linear law, able to address mixed-mode failure conditions. The derivation of the traction-separation law and its relation with potential-based laws is discussed. Upon interface failure, a non-linear frictional contact analysis is used, to address separation, sliding or sticking between micro-crack surfaces. To avoid a sudden transition between cohesive and contact laws, when interface failure happens under compressive loading conditions, the concept of cohesive-frictional law is introduced, to model the smooth onset of friction during the mode II decohesion process. The incrementaliterative algorithm for tracking the degradation and micro-cracking evolution is presented and discussed. Several numerical tests on pseudo-and fully three-dimensional polycrystalline microstructures have been performed. The influence of several intergranular parameters, such as cohesive strength, fracture toughness and friction, on the microcracking patterns and on the aggregate response of the polycrystals has been analyzed. The tests have demonstrated the capability of the formulation to track the nucleation, evolution and coalescence of multiple damage and cracks, under either tensile or compressive loads.

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Dynamic Embrittlement — Time–dependent Quasi–brittle Intergranular Fracture at High Temperatures

Cited by 85 publications

References 89 publications

Oxidation of a Commercial Nickel-Based Superalloy under Static Loading

Oxidation of a Commercial Nickel-Based Superalloy under Static Loading

Influence of overloads on dwell time fatigue crack growth in Inconel 718

A three-dimensional cohesive-frictional grain-boundary micromechanical model for intergranular degradation and failure in polycrystalline materials

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